Use a boning knife to remove the skin from the roast. Leave a layer of fat on the pork shoulder.
Once the skin is removed, poke the roast all over with the knife to create slits.
Smash the garlic, salt, pepper and oregano with a mortar and pestle, or blitz in a food processor.
Add olive oil to the paste, and stir to combine.
Slather all sides of the pork shoulder with the garlic paste, making sure to push some into the slits you created earlier. If you're adding sofrito, add it now.
Marinate overnight, or up to 24 hours. If you don't have time to marinade, don't worry. It will still be delicious.
Place the pork shoulder into a 6 quart slow cooker ceramic insert fat side down, and cook for 9-10 hours on low or 6-7 hours on high, until the meat shreds easily and the bone is completely detached from the meat. Shred the meat into chunks with 2 forks.
Freeze leftovers in a plastic freezer storage bag. Reheat in the microwave as needed.